A plasma display panel (hereinafter, referred to as a PDP) allows realization of a high degree of definition and a large screen, and therefore is for use in a large television set having a screen size of, for example, 65 inches or more. Recently, such a PDP has been applied increasingly to a high-definition television set having scanning lines the number of which is more than twice as large as those of a conventionally known television set adopting an NTSC system, and further reduction in cost thereof has been demanded.
A PDP basically includes a front plate and a back plate. The front plate typically includes a front substrate, a large number of display electrodes which are formed in stripes on one side of the front substrate, a dielectric layer which covers the large number of display electrodes and serves as a capacitor, and a dielectric protective layer which is formed on the dielectric layer. On the other hand, the back plate typically includes a back substrate, a large number of address electrodes which are formed in stripes on one side of the back substrate, and a base dielectric layer which covers the large number of address electrodes. Herein, a large number of partition walls are formed in stripes on the base dielectric layer. These partition walls are arranged in parallel with the address electrodes such that each address electrode is located between the adjacent partition walls when being seen in a thickness direction of the back plate. Moreover, the base dielectric layer and side surfaces of the adjacent partition walls form a plurality of grooves coated with a phosphor layer emitting red light, a phosphor layer emitting green light, or a phosphor layer emitting blue light in sequence.
In the PDP, the front plate and the back plate are arranged such that the display electrode formation side and the address electrode formation side face each other. Further, the PDP has an outer periphery sealed with a seal member. That is, the PDP is of an enclosed structure. In this enclosed structure, an enclosed space is formed and is filled with a discharge gas containing neon (Ne), xenon (Xe), and the like at a pressure of 53,000 to 80,000 Pa, so that a discharge space is formed. In the PDP, a video signal voltage is applied selectively to the display electrode, so that gas discharge occurs at the discharge space. Then, ultraviolet rays are generated by the gas discharge, and each phosphor layer is excited by the ultraviolet rays to emit visible light. Thus, a color image can be displayed on the PDP.
In the PDP configured as described above, the display electrodes are led as lead electrodes at predetermined intervals in a non-image display region located on an outer periphery of the front plate. This lead electrode is connected to an electrode of an external drive circuit of the PDP. It is to be noted that the non-image display region to be described herein refers to a region which intends to establish connections with the external drive circuit and the back plate, but does not mainly intend to display an image.
Patent Document 1 (Japanese Patent No. 3980462) discloses one example of the PDP having the configuration described above. In addition to the configuration described above, a conventional PDP disclosed in Patent Document 1 further includes a dummy electrode formed near lead electrodes in a non-image display region on an outer periphery of a front plate.
FIG. 15 shows a positional relation among electrodes formed on the outer periphery of the front plate of the conventional PDP.
As shown in FIG. 15, the front plate includes a front substrate 210, a large number of display electrodes 211 formed in stripes on the front substrate 210, and a large number of lead electrodes 211a led from the large number of display electrodes 211, respectively. The lead electrodes 211a are formed such that a clearance therebetween becomes narrow gradually, and are connected to corresponding terminals in a plurality of terminal groups 211b which are arranged along an edge of the front substrate 210 at predetermined intervals. Moreover, a different terminal 217 and a dummy electrode 218 are formed on a region located between the adjacent terminal groups 211b and 211b. The dummy electrode 218 is interposed between the different terminal 217 and each of the terminal groups 211b and 211b to insulate the different terminal 217 from the terminal group 211b.     Patent Document 1: Japanese Patent No. 3980462
Recently, a PDP has been subjected to various product developments in order to achieve further reduction in cost thereof. As one of the product developments, a method for forming a dielectric layer is improved for achieving such reduction in cost. Conventionally, a dielectric layer is formed so that a glass frit-containing paste is applied, dried and baked repetitively. According to the improved dielectric layer forming method, a sol with low viscosity, which is a colloid solution made of metal alkoxide, is used as a dielectric material. This sol is solidified by hydrolysis and polycondensation reaction to form a gel. Then, this gel is subjected to heat treatment to obtain an oxide. That is, the dielectric layer is formed by sol-gel reaction. The conventional dielectric layer forming method requires a process of baking the applied paste at a temperature which is not less than a softening point of glass frit. According to the improved dielectric layer forming method, on the other hand, the dielectric layer can be formed at a temperature which is lower than the conventional temperature by virtue of the sol-gel reaction. This improvement allows realization of reduction in manufacturing cost.
In the improved dielectric layer forming method, however, the sol, when being applied onto a front substrate, is apt to flow on the front substrate because of the low viscosity thereof. The flow of the sol depends on presence/absence of an electrode, so that a circumference of the dielectric layer does not become even (linear), but becomes wavy. The wavy circumference of the dielectric layer lowers a sealability of a seal member, leading to degradation in quality of a PDP. With reference to FIG. 16, this point is described in more detail.
FIG. 16 is a partly enlarged plan view showing the front plate of the conventional PDP in a state that a dielectric layer has a wavy circumference. It is to be noted that the front plate shown in FIG. 16 does not include the different terminal 217 and the dummy electrode 218.
It is assumed herein that a sol with low viscosity is used as a dielectric material and a dielectric layer 215 is formed by, for example, a die coat method of applying a sol discharged from a slit die. In such a case, the sol is apt to flow along the display electrode 211 and the lead electrode 211a. Herein, the plurality of lead electrodes 211a are formed such that a clearance therebetween becomes narrow gradually. As a result, the sol hardly flows toward a region 250 located between the adjacent terminal groups 211b and 211b. As shown by a dotted line in FIG. 16, consequently, waviness occurs at a circumference of the dielectric layer 215. Herein, there is a possibility that waviness occurs at the circumference of the dielectric layer 215 in a non-image display region (an upper region in FIG. 16) located beside the display electrode 211, depending on viscosity of a sol to be used, process conditions (e.g., an applying pressure, an applying rate) of an application method, and the like.
The wavy circumference of the dielectric layer 215 causes the following problems about a positional relation between the dielectric layer 215 and a seal member.
In the non-image display region on the outer periphery of the front plate, herein, the seal member is attached to the entire circumference of the dielectric layer 215. Thus, the front plate is connected to a back plate through the seal member such that a sealed space is formed between the front plate and the back plate.
As shown in FIG. 17A, most preferably, the dielectric layer 215 and the seal member 218 are arranged such that a part of the seal member 218 comes into contact with the dielectric layer 215 and the remaining part of the seal member 218 comes into contact with the lead electrode 211a or the front substrate 210. In contrast to this, as shown in FIG. 17B, if the seal member 218 comes into contact with only the dielectric layer 215, there may occur a sealing leak between the front plate 201 and the back plate 202 because the seal member 218 does not come into close contact with the lead electrode 211a or the front substrate 210. As shown in FIG. 17C, moreover, if the seal member 218 does not come into contact with the dielectric layer 215, there may occur a malfunction such as abnormal discharge because the lead electrode 211a is bared partly.
Accordingly, the wavy circumference of the dielectric layer 215 causes the disadvantages shown in FIGS. 17B and 17C, resulting in occurrence of the malfunctions such as the sealing leak and the abnormal discharge. In the former case, moreover, the seal member 218 is pressed unevenly at the time when the front plate and the back plate are joined together. This unevenness causes a variation of gaps in the PDP, resulting in degradation in quality of the PDP. In order to avoid these disadvantages, preferably, a waviness width at the circumference of the dielectric layer 215 (i.e., a length between an innermost wavy portion and an outermost wavy portion in the dielectric layer 215) must be suppressed within ±2 mm (i.e., 4 mm).
However, the configuration of the conventional PDP, for example, the PDP disclosed in Patent Document 1 fails to suppress the waviness width at the circumference of the dielectric layer and accordingly fails to avoid the respective malfunctions.
The present invention has been devised to improve the issues described above, and an object thereof is to provide a front plate for PDP, in which a circumference of a dielectric layer can be made even although the dielectric layer is made of a material with low viscosity, and a method for manufacturing the same, as well as a PDP including the front plate.